This invention relates to a photothermographic image recording element, and more particularly, to a photothermographic element suitable for use in a photomechanical process and especially adapted for scanners and image setters. More specifically, it relates to such a photothermographic element exhibiting excellent photographic properties including low Dmin and high contrast.
There are known a number of photosensitive elements having a photosensitive layer on a support wherein images are formed by imagewise exposure. Among these, a technique of forming images through heat development is known as a system capable of simplifying image forming means and contributing to the environmental protection.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the photomechanical process field to reduce the quantity of spent solution. Needed in this regard is a technology relating to photothermographic elements for use in photomechanical processes which can be effectively exposed by means of laser scanners or laser image setters and produce distinct black images having a high resolution and sharpness. These photothermographic elements offer to the customer a simple thermographic system that eliminates a need for solution type chemical agents and is not detrimental to the environment.
The technology of forming images through heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d in xe2x80x9cImaging Processes and Materials,xe2x80x9d Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic elements generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an organic binder matrix. Photothermographic elements are stable at room temperature. When they are heated at an elevated temperature (e.g., 80xc2x0 C. or higher) after exposure, a redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the reducible silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Preferred among the organic silver salts are silver salts of organic acids. For the preparation of organic acid silver salts, it is well known in the art to treat an organic acid such as an aliphatic carboxylic acid with an alkali to form the organic acid alkali metal salt, and add silver nitrate thereto, whereupon an exchange reaction takes place between the alkali metal of the organic acid alkali metal salt and silver ion to thereby form the organic acid silver salt. This process, however, has difficulties obtaining homogeneous organic acid silver because the organic acid alkali metal salt becomes solid, which hinders uniform proceeding of the exchange reaction with silver ion.
It is described, for example, in Journal of Japanese Photographic Society, vol. 52, page 21 (1989), to form a homogeneous solution of an organic acid alkali metal salt in the co-presence of an alcohol during preparation of organic acid silver. As used therein, a secondary alcohol such as isopropanol is insufficient in fog. If a primary alcohol such as ethanol is used, it can react with silver ion to form explosive silver fulminate.
EP 762,196 and JP-A 90550/1997 disclose introducing metal ions or metal complex ions belonging to Group VII or VIII (Groups 7 to 10) in the Periodic Table into photosensitive silver halide grains to be used in thermographic image recording elements and to introduce hydrazine derivatives into photosensitive elements to achieve high contrast photographic properties. In either case, silver bromide is used as the photosensitive silver halide. When these elements are applied as printing plates, their high Dmin or fog in the UV region in which the wavelength of a light source for use in the exposure of printing plates falls is a problem. The Dmin in the UV region can be reduced by converting the silver halide to a high silver chloride content one. This, in turn, raises the problem that the element becomes susceptible to fog and fails to achieve a high contrast and high Dmax.
Therefore, an object of the invention is to provide a photothermographic image recording element suitable for use in a photomechanical process, exhibiting excellent photographic properties including a high contrast and low Dmin or fog so that it is especially adapted for scanners and image setters, and featuring consistent manufacture. More specifically, a first object is to provide a photothermographic image recording element exhibiting a low Dmin or fog; and a second object is to provide a photothermographic image recording element exhibiting a low Dmin or fog, high contrast, and high speed.
In a first aspect of the invention, there is provided a photothermographic image recording element comprising a non-photosensitive organic silver salt and a photosensitive silver halide on a support. The non-photosensitive organic silver salt has been formed in the presence of a tertiary alcohol. A photosensitive layer containing the photosensitive silver halide or a layer disposed adjacent thereto or both contain a nucleating agent.
In a second aspect of the invention, there is provided a photothermographic image recording element comprising on a support a photosensitive layer containing a non-photosensitive organic silver salt, a photosensitive silver halide, and a binder. The photosensitive layer has been formed by applying a coating solution in which water constitutes at least 60% by weight of the solvent. The non-photosensitive organic silver salt has been formed in the presence of a tertiary alcohol. The photosensitive silver halide has been formed independent from the non-photosensitive organic silver salt and added during preparation of the coating solution. The binder contains at least 50% by weight of a polymer latex having a glass transition temperature of xe2x88x9230xc2x0 C. to 40xc2x0 C. Preferably, the photosensitive layer or a layer disposed adjacent thereto both contain a nucleating agent.
In the elements of the first and second aspects, the nucleating agent is preferably selected from the group consisting of substituted alkene derivatives of the following formula (1), substituted isoxazole derivatives of the following formula (2), and acetal compounds of the following formula (3): 
wherein R1, R2, and R3 are independently hydrogen or substituents, and Z is an electron attractive group or silyl group, and at least one pair of R1 and Z, R2 and R3, R1 and R2, and R3 and Z, taken together, may form a cyclic structure; 
wherein R4 is a substituent; 
wherein X and Y are independently hydrogen or substituents, A and B are independently alkoxy, alkylthio, alkylamino, aryloxy, arylthio, anilino, heterocyclic oxy, heterocyclic thio, or heterocyclic amino groups, and X and Y, and A and B, taken together, may form a cyclic structure. Alternatively, the nucleating agent is a hydrazine derivative.